Ceramic burner plate



Feb. 23, 1965 J. G. LANNING 3,170,504

cERAMI BURNER PLATE Filed June 5, 1962 3 Sheets-Sheet 1 a 22. N311 l a x /1 I la 3o ll 4 f l /f l/ ,l I, f, l, I a? x Q a n 44 o V/yf//V/ /%7/%//7//,

(I m f l 6 INVENTOR.

3o 25 45 25g f/H/v G. ANNI/VC TTENEY Feb. 23, 1965 Filed June 5, 1962 J. G. LANNING CERAMIC BURNER PLATE 5 Sheets-Sheet 2 n n n a l f a n 4 l e v mam/@ INVENTOR. John 6. Lann/'ng ATTORNEY United States 'Patent -3,170,504 CERAMICl BURNER PLATE John G. Lanning, Corning, N.Y., assigner to Corning Glass Works, Corning, N Y., a corporation of New vYork Filed June 5, 1962, Ser. No. 200,235 S Claims. (Cl. 158-116) This invention relates to ceramic burner plates for infrared radiating gas burners and the like. In particular, it comprises a burner plate having an improved and novel combustion surface configuration or geometry.

Heretofore, ceramic burner plates have been constructed in the form of a perforated or honeycomb structure having a plurality of small unobstructed gas passages or open-ended cells separated from each other by thin ceramic walls. The gas passages extend through the burner plate from one major face of the plateto an opposed or opposite major face thereof. One of these major faces constitutes an external combustion surface when the plate is placed over an -openingin a gas burner mixing chamber in such ay manner that the other major face confronts the interior of the mixing chamber.

States Patent No. 2,775,294 to- G. Schwank.

It has been found that the fiat combustion surface of these burner plates is not uniformly heated by the burning fuel because of slight differences in the thickness of the cell walls and hydraulic diameters of the cells, which is an inherent result of the methods of manufacturing these plates. tion variation to cause uneven heating and, as a result,

the flat surface has a mottled appearance. This mottled` surface appearance'varies considerably in temperature from the orange-white high temperature areas to `the dullV red or grey low temperature areas. Such a surface condition is undesirable since the maximum conversion of the available VB.t.u. content of the gas into infraredenergy` is not being achieved.

It is Aan object of this invention to provide a ceramic burner plate having a novel combustion surface configura- .tion or geometry whereby the thermal gradients or uneven heating of the radiating combustion surface due to slight differences in the thickness of the cell walls and hydraulic diameters of the cells is eliminated or greatly reduced. 7

It is another object of this invention to provide a ceramic burner plate having a novel combustion'surface configuration or geometrywhereby much moreV eflicient combustion is effected by the resulting thermocatalytic combustion. A

' It is a further object of this invention to provide a ce ramic burner plate having a novel cumbustion surface configuration or geometry which affords a ymaximum realization of the available heat content of the gaseous fuel and thereby results in increased temperature of the radiating combustion surface and in greater burner efficiency.

It is a still further object of this invention to provide a ceramic burner plate having a novel combustion surface configuration or geometry that affords an increase in the effective infrared radiating'surface area without increasing the overall size of `the burner plate, which results in a more efficient combustion of thegaseous fuel and in These differences create sufficient' combusl 3,170,504 Patented Fei. 2,3, 1965 ICC a more uniforminfrared` heat radiation over the entire combustion surface. t

These and other objects, which will become apparentv to those skilled in the art from the detailed description given` hereinafter and from the accompanying drawings, are attained 1n accordance with-my discoveries by providing va ceramic gas burner plate comprising a thin Ywall ceramic,

perforated or honeycomb structure having two opposed,

major faces with a plurality of small gas flow passages extending between the opposed major faces and with one major face comprising a plurality of substantially uniform or purposely irregular peaked protrusions, which may be generally of pyramidal or conical shape, disposed substantially uniformly or purposely irregularly over substantially the entire area or a given portion of this one major face. As a general rule, it is preferred to provide the peaked protrusions with substantially pointedV FIGURE l is a sectional oblique view of an infrared gas,l burner with one embodiment of a burner f plate of; the

invention;

FIGURE 2 is a sectional view of an infrared gas vburner like that shown in FIGURE 1A taken along line FIGURE 3l is a sectional isometric View of another j embodiment of ya burner plate of the invention;

FIGURE 4 iis a fragmentary Aside sectional View of al I ,burner plate'of the invention;

to the present invention. The zone 20 within thev body FIGURE 5 is a fragmentary'side sectional View of a burner plate of the type pri/or to this invention; Y

Y FIGURE 6 is an enlarged fragmentary sectional viewj of the burner plate structure shown in FIGURE 2'; andA walls 12 and a bottom Wall 1.4. Attached to body mem ber El? is a centrally disposed inlet ,tube 16 extending from one side wall `12. The top surface or cover pla-tel of the gas burner is a ceramic honeycomb structure 18' comprising one embodiment of a burner plate accord-ing tube 16 and prior to passing upwardly through the cover plate 18 for combustion on vthe upper surfacethereof.

The honeycomb burner plate 18 is fitted and attached to the ibody 10 at the upper portion of -t-he side wall's'plZ by any suitable means, such as cement or packing 22g` As is shown in the drawings, the honeycomb burnerA plate 18 is characterized lby aV large number of unobstructed gas paths'or *open-ended cells Z5 that extend from the bottomV or inlet surface 27 of the burner platev through Ito its top or combustion surface 28. These unobstructedgas Ipaths are defined and Aseparated from oneV another by thin ceramic walls 30. By unobstructed, IA

mean that no structure exists within the vgas flow paths* that would prevent flow. The ceramic walls defining those flow paths can be arranged triangular, circular or polygonal .shape as desired.

show p volumeofrgaseous fuel per unit of time.

Vhigher surface temperature promotes therrnocatalytic perature.

f or purposelyirregular peakedprotrusions disposed substantially Auniformly or purposely irregular 'over substantially the entire area of, or a given -portion of, surface Z8, as shown in FIGURES 1 and 4. These peaked protrusions are shown to be pyramidal shape in'FIGURE l, but they can also be conically shaped, if desired,A as shoWn inl-FIGURE 3. In this latter embodiment, thejhoney- A comb Iburner plate 3l. ischaracterized by a large number of unobstructed gas'ipaths 25 defined and'separated from one another by thinceramic walls 34. These gas paths extend from the bottom or inlet surface 32 through .to the top or combustion surface 33. y

In the use of a burner plate of the inventioma source of a gaseous fuel and air enter theburner body lil through the inlet tube l2 or any other suitable inlet means. The fuel and air can be provided `by sources connected to a mixing head `thatis attached lto the inlet means, or the pressure of the fuel gas vcan be used with suitable Vmeans to aspirate air 4into the stream.V

Other procedures,` none of which form any part of the invention, can be used as well. The .gases enter the enlarged space within the burner body l0 and thorough- Y ly -mix therein. For mixing purposes, the inlet tube can be designed sothat the gases impinge on a wall of the body member-10, orAbaliies (not shown) can be included within the `'bodyuor tube to eiect'that object. The rnixed gases then pass through the gas paths 2S in the burnerplate tothe upper surface 2S where they are combusted,heating' the surface of the rplate to nearly incandescence`.- yAny as desired.

The significant improvements affordeddby the vburner particle reinforcement. Thus, as shown-'in FIGURE '4;

With a burner plate according to this invention, having a theyhemispherical radiation of each surface particle will Y impinge on other surface particles on adjacent peaked protrusrions. Thiseifect, which'l call interparticle rein-V forcement, resultsin increasing the. overall temperature radiating combustion surface dueto the slight differences'v in the thickness of the cell walls and hydraulic diameters ofthe cells. This problem'can best be understood by rreference to FIGURE 6. The honeycomb structure is made of thin ceramic sheets 3i)V sintered together as at points to form Vthe thin Walls'denirxgnthe gas paths ceramic walls makingnp aflat combustion surface. How- Y ever, with la peaked combustionjsurface burner plate of this invention, the hemispherical reinforcement'radiation Vfrom adjacent peaked protrusions causes these'thickervr wall edge points 45 to heat up more uniformly Withlall other areas on the combustion surface.` It can also be seen in FIG. 6 that if the-cross-sectional areas of the Vgas passages vary one from another (eg. if the cross-sectional Y area of passagek 25a is somewhat larger than,` that of' passage ZSb), the hydraulic radius thenfvar'iesand the gas mixture velocity through thesepassages Yalsoyaries since the pressure drop from the inlet end of the.V passages to the outlet end of the passages is the'same,V for each'passage., This variation in the velocity of the gas/ air mixture causes an uneven burning condition atzthel combusf tion surface. This results in unevenV surface temperatures and the characteristic mottled appearance of thecombustionV surface. However, fwith a Ypeaked combus- 1 tion surface burner plate, this tendency towardrunevenk Vsuitable. conventional combustion initiator can'be used e plate'of the invention are believed .to be dueto interv' combustion surface comprising a Vmultitude of peaked protrusions it' can be seen that a substantial Vamount of of the vcombustion .or radiating surface 28 for any given Y In turn, Vthis combustion and more complete combustionof the gaseousv fuel thereby liberating maximum heat content from the fuelto further increase thencombustion surface tem- Y As willfbe apaprent, the higher surface tem- 1 peratures, providedby the burner; plate of the invention, increase theV amount of radiant'energy relative to convccted energy for any given unit volume of gaseous fuel combusted; 'thus the invention affords Ya significantly greater infrared burner efficiency. Moreover, .the geometry orconiguration ofthe peaked protrusionsfprovide a4 substantially increased area for radiation lof infrared energy from the combustion surface thereby producing a more v.uniform and widerV dispersion ofv infrarred heat l. angle d (see FIGURE 4)' relationship lbetween adjacent pyramidal or conical surfaces.` The angler)V should be beemission from the combustion surface of the burner Y plate.V

40. A surfaceL particle 41 radiates energy in 'thev hemispherical pattern as shown at `42.1 As-can be seen, suba stantially none of thehemispherical Yradiation impinges onother surface particles. Thus, Vinterparticle reinforce;- f-

menty is substantially whollyabsent.

n As was mentioned earlier, a particularly undesirable problem was the' development ofth'errnal, gradients on `the surface temperatures, dueto slight variations in the hydraulic diameter of thegas/air'rnixture flow passages, is substantiallyk reduced.` This 'Iris substantially a result of the interparticle vreinforcement described above.

.. Y AThe areas where the hydraulic diameter is lessv thanthe normal (and on a fiat combustion Vsurface results in theVV darker color and lowerrtemperature) interparticlerein` Y forcement'dueptoy a peaked `combustion.surface raises the temperature. This results because Vthe total radiation from the hotter areas lis greater than that'from the cooler areasV as taught by the StefanfBoltzrnannclaw.l Thus, since in a rpeakedl combustionsurface,A the cooler areas are seen by the hotter area s,the cooler lareas receive radiation energy yfrom the hotter areas and the'temperature differentialbetween the vtwo areas is substantially reduced. Y t .Y

The combustion surface configuration of' burner plates Y according to thisinvention should have certain particular characteristics that kare necessaryV for practical results. One important characteristic is an appropriate :included tween about 20 and 120, and preferably between 30 and The significance-.of'this .angular relationship can best be seen by reference toFIGURE 7 wherein comv bustion surface temperature data isshown as a function of the included angle 0.` The.V data 4was obtained by solely varying the angle 0 and maintaining all other factors (eg. burner plate dimensions, fuel, etc.) `the same.. The temperature measurements were -rnade by focusing a radiant energy detecting pyrometer (see FIGURE 4)y on the combustion surfacej at ananglefoc of 30," fto the normal `of the general plane yofgthat. sUIface.'I-`he plot of data for an angle fx ofy 30 to VthenormalV is shown in FIGURE 7 as merely one convenient illustratiomof the pattern of variation in combinationv surface temperature withfvariationof the' included Vangle 0. `Similar pat- .tems of variation betweencombu'stion surfaceftemperature and included angle 0 are found rwith an'angleu of 0 to 60. Thus, asY Vshown in FIGURE V,7, angles of greater than.` about do not produce enoughinter.-

' particlereinforcementfor practical Yresults of ,increased combustion surface temperature and of maximizing com- Vvbustion ofthe gaseous fuel.` Angles substantially smaller lthan 20 although having acceptablerperformance bev Y comeV impractical to Ar'nanufactureand usegbecause ofthe voverall depth fof the'burnerrplate .(asl will be apparent ESI 3 from the further discussion below) and fragility of the peaks.

Another important characteristic of the combustlon surface geometry is 4the spacing between apexes of adjacent peaked protrusions, which governs the depth of the valley portions between the peaked protrusions. This spacing between adjacent .apexes should be at least 1/8 inch, and preferably between l/i to 1/2 inch. A spacing of 1A inch was used in burner plates from which the plotted data of FIGURE 7 was obtained. v

The minimum spacing of 1/s inch appears to be a limit at which signiiicantly practical results' are obtained, and below which improvements are too insubstantial to be noticeable, The spacing can be made as much as one inch or more but the distance between the peaks becomes -so great that -interparticle reinforcement is substantially decreased and performance reduced.

Ceramic honeycomb or perforated structures that are used for burner plates in accordance with the present invention should have certain particular characteristics that are necessary for proper operation without incurring detrimental ashback. First, the gas paths 25 should have a cross-sectional area at the combustion surface that does not exceed 0.006 square inch and a minimum length of about 0.2 inch, therlatter being the minimum distance between surface 27 and the lowermost part in the Valleys between peaked protrusions or surface 2S. The gas passages, of course, should be substantially uniformly distributed throughout the burner plate and their crosssectional areas in the aggregate should provide at least 20%,' and more preferably at least 50%, of the total area f of the combustion or radiating surface. When this total aggregate open area is or more, the ceramic walls will necessarily be quite thin (i.e. 0.0l inch or less) and the upwardly flowing gas will extract most of the heat A conducted down the wall of most any suitable ceramic material used, thereby preventing the well known undesirable elfect of flashback from occurring. In the case where the total aggregate vopen area is less than 50%, with correspondingly thicker walls, the ceramic material used for the burner plate should desirably have a low heat conductivity on the order of less than 0.0020 cal./ sec. cm. C. since the greater mass of the walls makes it less possible for the upwardly flowing gases to extract enough heat from higher heat conductivity ceramic walls to prevent flashback.

By Way of illustration, and not of limitation, the following is given as the best mode lcontemplated of making burner plates according to this invention. A ceramic honeycomb body is prepared by coating a suitable carrier with a mixture of a pulverized ceramic and a binder, crimping the resulting coated carrier and then assembling it to the desired shape, alone or with `another coated carrier that need not be crimped. VThe assembled body is then heated to a temperature sufficient to sinter it to a unitary structure as more fully detailed hereinafter. This procedure is, generally, the process yset forth in the copending application of R. Z. Hollenbach, Serial Number 759,706, tiled September 8,1958, and now Patent No. 3,112,184, to which reference may be had.

The purpose of the binder is to bond the uniired ceramic material to the carrier, to impart green strength to the coated carrier and to retain the formed unred article in the desired shape after forming and prior to sintering. It is preferred to use an organic binder that is curable or thermosetting and that can be removed by decomposition and/or volatilization when the honeycomb body is tired, such as epoxyresins.

The purpose of the carrier is to provide support for the unired coating to allow it to -be formed to the desired shape prior to sintering the ceramic coating. Tea bag paper is a preferred carrier because it will substantially decompose upon tiring and thus result in an article consisting almost entirely of ceramic material.

Other suitable binders and carriers are disclosed in the mainder petalite.

aforementioned Hollenbach patent application, to which reference can be made.

In order to produce a burner plate structure with optimum desirable characteristics of strength, low coeicient of thermal expansion (eg. -10 to +20 107/C.), thermal shock resistance and high specific heat, it is preferred to use lithium aluminosilicate ceramic materials such, for example, as glass or crystalline petalite and beta spodumene, glass-ceramics having a lithium aluminosilicate base (erg. those made in accordance withV Example l, of United States Patent Number 2,920,971 to Stookey), aswell as mixtures of any of the foregoing materials.

Petalite glassceramic mixtures generally include about 10 to 40 weight percent of the glass-ceramic and the re Beta spodumene-petalite mixtures usually contain about l to 4 parts of petalite for each 4 to 1 parts of beta spodumene. These materials normally are used in a particle size of about minus 200 mesh (Tyler) or finer.

Structures are assembled from ceramic coated carriers in a Variety of ways, and the resulting structures are called layers laterally'disposed a distance equal to half of thek width of the individual pattern so.. that layers do not nest with each other. They can also be made from multiple layers of corrugated ceramic coated carriers with alternate layers having the corrugations angled in opposite direc-y tions from the perpendicular tothe edge of the sheet. The honeycomb structure'can also be formed from r-olling up alternate layers of crimped 'and uncrimped coated Acarriersuntil the desired shape is'formed. The structure can also be formed by assembling to a stack alternate crimped and uncrimped coated` carriers until the desired dimensions are attained. The structure is made large enough to form the peaked protrusions von one major surface, as by cutting or sawing, and to provide ya mini# mum gas ow path length of at least 0.2 inch. Other ways of making and assembling these honeycombs will be apparf ent to those skilled inthe art.

The tiring of the green structure or -matrix, however formed, is accomplished inthe normal manner for ceramic tiring by placing the structure in a furnace and heating it at a rate slow enough to prevent breakage up to a temperature high enough to cause the ceramic particles to sinter. While the tiring schedule, including heating rates and sintering temperatures, will vary depending upon the ceramic material utilized, vthe size and shape of the structure formed, and the atmosphere used, the details of such schedules are not critical and suitable conditions are readily determinable by one skilled in the art of tiring ceramic article. Y l

A The invention will be described further in conjunction with the following example in which the details are given by way of illustration and not by way of limitation.

A ceramic composition was made of parts by weight of petalite and 25 parts by weight of a glass-ceramic having the following approximate composition, by oxide analysis, in weight percent: 70% SiO2, 18% A1203, 5% TiOz, 3% LiOz, 3% MgO and 1% ZnO. The composition was ball-milled to a minus 200 meshtTyler) par-k of isopropanol of ethyl-acetate of Versamid of Hysol 6111 Versarnid 115 is the trade name of a thermoplastic polymer supplied-'by General Mills, lne. It is prepared by condensation of polymerized,unsaturatedfatty acids, such y as dilinoleic acid, with aliphatic amines, such as ethylene` diamine. Hysol 6111 is the trade name of an epoxy resin solution, supplied by Houghton Laboratories, Inc., containing 57% by weight of epoxy resin having a viscosity of about 2.5-4.0 poises at C., and epoXide equivalent (grams of resin/containing 1 v g. chemical equivalent Yof epoxy) of 595150, and a melting range of 73-85 C.

The ceramic material and the binder were further ballthat, although the present invention has been described" with respect to. specific details of certain embodiments thereof, it'is not intended that such details be limitations upon the scope of,V the invention,A exceptl insofar as .setv

forth in Vtheappended claims. Y Y,

Y, Vlhat is claimedis: V. i t

l. A gas burner plate comprising a thin walledcerarnic honeycomb having an inlet surface and an opposed combustion surface, aplurality of kunobstructed gas passages milled for about three hours to produce a uniform `sust pension. A porous natural cellulose paper, commonly known as 31/2 pound tea bag paper, cut'to a Width of 4 inches was then dipped into the suspension and dried by heating to 120 C-. for 2 minutes. The dried, Y coated paper was then heated to ,180 C. and crimped to produce v nels of the annular cylinder as they are formed during the roll-up operation. l

The unred honeycomb body is then placed-in a fur- Y nace'chamberv and heated in accordance with the following schedule:

Temperature range: Firing rate Room temp, to 700.y C. 350-C./hr.

Hold at 700 C. lhour.

V700." C. to `1220Ci Furnace rate.

Hold at 12207 C. 30 minutes.

Cool to room temp. Furnace rate. Retire toy 1240 C. 300 C./hr. Hold at 1240or C. 7 hours.

Thereafter the honeycomb'is cooled to handlingrtemperature. l p Y Burner plates were then sawed from ,the above honeycomb structure with a minimum gas flow path length 0f about :is inch and with square .base pyramidalpeakedV protrusions having a 1A inch lspacing between adjacent apexes. zSornewere made with a included angle between adjacent'pyramidal surfacesand others with a 45, 60, 75, 90, 105 and .120 included angles.

lThese burner plates were cemented to a `ceramic burner body as shown in FIGURE 1. v

Ceramic infrared gas burner plates prepared as just Vdescribed were actually tested and found tobefree from dark or cool zones'on the Vradiatingv surface. The radiating surface temperatures were significantly higher (see FIGURE 7) and the heat radiation ywas more uniform y than the previously made flat combustion surface ceramic burner plates. Moreover, significantly greater comb ustion of the gaseousfuel, i.e. natural gas and air, was also obtained as was evident by the .considerable reduction of the CO content of theV combustedY gases being given-V off from the burner plate. Y Y

In accordance with the provisions of the patentstatutes, I have explained'the principle of my invention and have illustrated and described what Inow believe to represent its best embodiment. However, it shouldfbe understood a pattern, taken in cross-section, in the shape of a triangle 1 j ofV peaked protrusions, `adjacent peaked protrusions hav-V in'gadjacent sloping surfacesforming an included angle,

uniformly. distributed throughout the honeycomb-and.

extending between and terminating in said surfaces, .said

passages being detined and separated from one another by thin walls of ceramic, each' said passage having a crossfsectionalarea not exceeding V0.006 square inch, the aggregate cross-sectional areas of the gas passages providing an open spacelofat least V20% of the area of said combustion surface, at least a portion of said combustion surface having a conguration. comprising a plurality therebetween of about20 to 120 and having apeXes spaced at least a distance of lsfinch whereby a plurality of said gas passagesterminate in the' sloping surfaces of each said peaked protrusion, andthe length of all said gas passagesbeing at leastaboutaOZginch. l n t Y 2. A gas burner plate according .to claim 1 in4 which the peakedprotrusions arepyramidal shape.

3, A gas burner plate according to claim 1 inwhich the peaked protrusions; are conical shape.

uniformly distributed kthroughout the honeycomb and extending lbetween andterrninating in saidV surfaces, said passagesbeingdened land separated from one another` by thin walls ofrceramic, each said passage having a crossv sectional` area not exceeding 0006 square inch,l the aggregate cross-sectional areas ofV they gas passages providing an openspace of atleast 50% fof the area'of said combustion surface, said combustion surface, havingaconfiguration comprising-,a plurality Yof'pealred protrusions,

'adjacent' peaked protrusions housing Y adjacent sloping surfaces forming an! included angle therebetween of about 307 to 90 and havingrapexes spaced Ya distance of about 1A to 1A. inch whereby a plurality `ofsaid gas passages Vterminate in the sloping surfaces of each said peaked protrusion, and the length of all Vsaid gas passagesV being at least about 0.2 inch;

5. A gas burnerplate accordingto vclaim 4 in whichl the peakedprotrusions are pyramidal shape. Y

6. A gas burner plate according toclaim 4-V in which the peakedprotrusions are vconical shape.-

7. .A gas burner plate according yto claim 4 in which the ceramic honeycomb is formed ofa lithium'aluminosilicate material.

8. A gas burnerk plate according toclaim 5 in.Y which the ceramic honeycomb is'formed of alithiumaluminosilicate material.

References Citedgin the tile of this' patent UNITED STATES PATENTS V1,731,053 L0we f f ocf. s, V1929 e Y l EonEroN PATENrs c u 551,940 Belgiuml Nov. 14, 195e 558,007 Belgium l June 29, 1957 626,559

Cerrada A Aug. 29, 19.51

4. A gas burner plate vcomprising a thin Walled ceramic .s 'honeycomb having aninlet surface and an opposed com- Y bustitmsurface,l a plurality of unobstructed gasrpassages UNITED STATES PATENT OFFICE CERTIFICATE oF CoRRECTloN Patent No. 3,170,504 February 23', 1965 John G'. Lanning It is hereby certified-that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line. 53, for "apaprent" read apparent column 5 line l2 for "obtained" read attained f line 27, for "or". read on column 8, line 42, for "housing" read having Signed and sealed this 24th day of August 1965,

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER1 Attesting Officer Commissioner of Patents 

1. A GAS BURNER PLATE COMPRISING A THIN WALLED CERAMIC HONEYCOMB HAVING AN INLET SURFACE AND AN OPPOSED COMBUSTION SURFACE, A PLURALITY OF UNOBSTRUCTED GAS PASSAGES UNIFORMLY DISTRIBUTED THROUGHOUT THE HONEYCOMB AND EXTENDING BETWEEN AND TERMINATING IN SAID SURFACES, SAID PASSAGES BEING DEFINED AND SEPARATED FROM ONE ANOTHER BY THIN WALLS OF CERAMIC, EACH SAID PASSAGE HAVING A CROSSSECTIONAL AREA NOT EXCEEDING 0.006 SQUARE INCH, THE AGGREGATE CROSS-SECTIONAL AREAS OF THE GAS PASSAGES PROVIDING AN OPEN SPACE OF AT LEAST 20% OF THE AREA OF SAID COMBUSTION SURFACE, AT LEAST A PORTION OF SAID COMBUSTION SURFACE HAVING A CONFIGURATION COMPRISING A PLURALITY OF PEAKED PROTRUSIONS, ADJACENT PEAKED PROTRUSIONS HAVING ADJACENT SLOPING SURFACES FORMING AN INCLUDED ANGLE THEREBETWEEN OF ABOUT 20 TO 120* AND HAVING APEXES SPACED AT LEAST A DISTANCE OF 1/8 INCH WHEREBY A PLURALITY OF SAID GAS PASSAGES TERMINATE IN THE SLOPING SURFACES OF EACH SAID PEAKED PROTRUSION, AND THE LENGTH OF ALL SAID GAS PASSAGES BEING AT LEAST ABOUT 0.2 INCH. 